The invention relates to a phase-adjusting device for the inverter, having controllable semiconductors, of a parallel-resonant circuit frequency converter having an oscillator which supplies control pulses to the semiconductors and is operated by a phase-comparing stage, such phase-comparing stage so controlling the oscillator frequency in dependence in the phase shift of a signal in phase with the inverter output voltage and a signal in phase with the inverter output current that the inverter output voltage and the inverter output current are in phase.
If merely the inverter output voltage were to be used to operate the semiconductors of the inverter, in order to maintain the oscillation of a loaded parallel-resonant circuit of a frequency converter, the result would be an inductive phase shift between the inverter output voltage and the inverter output current. The inductive reverse currents would endanger the semiconductors to be controlled.
The use of a known phase-adjusting device of the kind specified obviates this risk. A direct phase comparison of the inverter output voltage and the inverter output current enables a control signal for the oscillator to be obtained by means of which the phase shift can be controlled by raising or lowering the oscillator frequency. However, problems arise with that phase-adjusting device if a hardly detectable inverter output current, if any, is flowing. Such conditions occur, for example, during build-up.